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The Petrology of the Rotoiti Eruption Sequence, Taupo Volcanic Zone: an Example of Fractionation and Mixing in a Rhyolitic System 总被引：4，自引：0，他引：4
The Rotoiti eruption from the Taupo Volcanic Zone (TVZ) in northernNew Zealand produced voluminous pyroclastic deposits. The ferromagnesianmineral assemblage in these dominantly consists of cummingtonite+ hornblende + orthopyroxene with uniform magnesium/iron ratios;a second assemblage of biotite + hornblende + orthopyroxene,also with uniform Fe/Mg ratios, appears midway through the eruptionsequence and, thereafter, increases in abundance. These contrastingmineral assemblages, together with pumice clast and groundmassglass compositions, provide evidence for mingling of two discretemagmas. Similarities in the chemical characteristics of thetwo magmas suggest that they developed from a similar source.The eruption initially tapped relatively homogeneous magma thatwas erupted throughout most of this phase of activity. The middlestages of the eruption included some mixed magma. The finalstages of the eruption were dominated by a second magma composition,which was probably injected into the bottom of the main magmabody as the eruption proceeded. The source that fed the eruptionwas complex, and discrete magma bodies existed and evolved separatelyprior to the eruption. We conclude that eruptions in the TVZare fed from a diffuse upper-crustal zone of partially interconnected,and at times physically separate, magma bodies rather than fromcentralized and necessarily large long-lived magma chambers. KEY WORDS: Taupo Volcanic Zone; Okataina Volcanic Centre; Rotoiti eruption; rhyolite system; magma mixing 相似文献
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The climatology of the troposphere–stratosphere–mesosphere model of the Institute for Numerical Mathematics (INM) with the uppermost level at 0.003 hPa is presented. This model is vertically extended from the upper level of 10 hPa for the earlier version, and a drag parameterization due to internal gravity waves (GW) is included. The model describes the main features of the mesospheric circulation: decreasing and reversion of westerly and easterly winds, equatorward shift of the westerly wind maximum with height and reversal of the meridional temperature gradient in the upper mesosphere. The model underestimates to some extent the amplitude of wave number 1 for stationary waves in the winter hemisphere. The same holds for the internal low‐frequency variability in the winter stratosphere. The sensitivity of the model climate is studied with respect to the inclusion of orographic gravity wave drag and the variation of the source height of internal gravity waves. 相似文献
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Guanling Biota: A Marker of Triassic Biotic Recovery from the end-Permian Extinction in the Ancient Guizhou Sea 总被引：1，自引：0，他引：1
After a slow recovery from the end-Permian extinction during the Early Triassic and rapid radiation in the Middle Triassic, evolution of organisms reached a new peak phase in the Late Triassic. The Guanling Biota from the Wayao Member （conodont Paragondolella polygnathiformis Zone）, Falang Formation, Xinpu, Guanling County, Guizhou Province, southwestern China corresponds to this peak that marks the full recovery from the end-Permian extinction of marine ecosystems. The biota is of high diversity, containing well preserved and completely articulated skeletons of vertebrates comprising marine reptiles, fishes, and invertebrates including crinoids, ammonites, bivalves, and other fossils, and is one of the best examples of marine ecosystem records in life history. The fossil marine reptiles and crinoids are most significant in this biota, especially the marine reptiles, which provide an important link between the Triassic Pacific and Tethys, and between Triassic basal forms and the Jurassic-Cretaceous marine top predators. The most remarkable fossils are the large completely articulated ichthyosaur skeletons up to and more than 10 m, and the first recorded thalattosaurs and placodonts in China. Following our review, of the 17 named reptilian taxa the eight listed here are considered to be valid： three ichthyosaurs （Qianichthyosaurus zhoui Li, 1999; Guizhouichthyosaurus tangae Cao and Luo in Yin et al., 2000, Guanlingsaurus liangae Yin in Yin et al., 2000）, three thalattosaurs （Anshunsaurus huangguoshuensis Liu, 1999, Xinpusaurus suni Yin in Yin et al., 2000, Xinpusaurus kohi Jiang et al., 2004）, and two placodonts （Sinocyamodus xinpuensis Li, 2000, Psephochelys polyosteoderma Li and Rieppel, 2002）. Mixosaurus guanlingensis Cao in Yin et al., 2000 might be a junior synonym of Qianichthyosaurus zhoui Li, 1999, and Cymbospondylus asiaticus Li and You, 2002 and Panjiangsaurus epicharis Chen and Cheng, 2003 might be junior synonyms of Guizhouichthyosaurus tangae Cao and Luo in Yin et al., 2000. It needs to re-describe the holotypes after a complete preparation for clarifying the taxonomic status of Typicusichthyosaurus tsaihuae Yu in Yin et al., 2000, Xinpusaurus bamaolinensis Cheng, 2003, Neosinosaurus hoangi （Zhou in Yin et al., 2000）, Wayaosaurus geei Zhou in Yin et al., 2000, Wayaosaurus bellus Zhou in Yin et al., 2000 and Placochelys ？ minutus Yin and Luo in Yin et al., 2000. 相似文献
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Anders LINDSKOG Birger SCHMITZ Anders CRONHOLM Andrei DRONOV 《Meteoritics & planetary science》2012,47(8):1274-1290
Abstract– Numerous fossil meteorites and high concentrations of sediment‐dispersed extraterrestrial chromite (EC) grains with ordinary chondritic composition have previously been documented from 467 ± 1.6 Ma Middle Ordovician (Darriwilian) strata. These finds probably reflect a temporarily enhanced influx of L‐chondritic matter, following the disruption of the L‐chondrite parent body in the asteroid belt 470 ± 6 Ma. In this study, a Volkhovian‐Kundan limestone/marl succession at Lynna River, northwestern Russia, has been searched for EC grains (>63 μm). Eight samples, forming two separate sample sets, were collected. Five samples from strata around the Asaphus expansus–A. raniceps trilobite Zone boundary, in the lower‐middle Kundan, yielded a total of 496 EC grains in 65.5 kg of rock (average 7.6 EC grains kg?1, but up to 10.2 grains kg?1). These are extremely high concentrations, three orders of magnitude higher than “background” levels in similar condensed sediment from other periods. EC grains are typically about 50 times more abundant than terrestrial chrome spinel in the samples and about as common as terrestrial ilmenite. Three stratigraphically lower lying samples, close to the A. lepidurus–A. expansus trilobite Zone boundary, at the Volkhov‐Kunda boundary, yielded only two EC grains in 38.2 kg of rock (0.05 grains kg?1). The lack of commonly occurring EC grains in the lower interval probably reflects that these strata formed before the disruption of the L‐chondrite parent body. The great similarity in EC chemical composition between this and other comparable studies indicates that all or most EC grains in these Russian mid‐Ordovician strata share a common source––the L‐chondrite parent body. 相似文献